1. Field of the Invention
The present invention relates to an image display apparatus having an optical system which combines original images into one composite image so that the original images displayed on multiple image display devices overlap one another and presents the composite observation image to a viewer and, in particular, to a technique for correcting distortion of the optical system.
2. Description of the Related Art
In recent years, an augmented reality technology or so-called Mixed Reality (MR) technology has become known that seamlessly merges the real and virtual worlds in real time. In the MR and virtual reality technologies, Head Mounted Displays (HMDs) are often used to provide mixed and virtual reality images. For example, one MR technology uses a video see-through HMD to present mixed reality images to a user. Specifically, a subject that is substantially identical to a subject viewed from the location of the pupils of a user of the HMD is captured by a video camera or the like, a CG (Computer Graphics) image is superimposed on the captured image, and the resulting image is displayed so that the user of the HMD can view the image.
FIG. 20 shows an HMD 900 being worn by an HMD user. The HMD 900 allows the HMD user to experience MR space.
An HMD requires an observation optical system capable of image display with a wide angle of view in order to provide a high degree of realism. In addition, an HMD needs to be lightweight and small so as not to cause fatigue to a user wearing the HMD on his/her head. In order to implement a small lightweight observation optical system capable of image display with a wide angle of view, a large single display device is desirable. However, there are few off-the-shelf large display devices that can provide a wide angle of view in a configuration of an observation optical system of a conventional HMD. Therefore, there is an observation optical system that uses multiple off-the-shelf small display devices, which are available in large quantities, each of which displays original images viewed in different fields of view, combines the multiple original images into one, and allows a user to view the resulting image. The image display form using such an observation optical system is called tiling display.
For example, an HMD using an optical device having five curved surfaces has been proposed. This HMD uses a prism having multiple sides to reflect light multiple times to fold the optical path in an optical device in order to minimize the size of the optical system. The optical system has a shape with five sides formed by joining three-sided prisms symmetrically about the visual axis. Image light rays from two display devices are internally reflected multiple times in areas in the prisms to fold the image light rays; image light rays of the halves of the angle of view are separately guided to the eyes through two areas. FIG. 21 is a diagram illustrating the optical system described in Japanese Patent Laid-Open No. 11-326820. The optical system includes first and second display panels 901 and 902 displaying divided images for tiling display and free-form surface prisms 903 and 904 guiding display images of the first and second display panels 901 and 902 to a viewing position.
Since more than one observation optical system, 903 and 904, is provided, distortion which occurs in the observation optical systems 903 and 904 due to differences in quality or in type between the individual observation optical systems 903 and 904 needs to be corrected for each display panel. Japanese Patent Laid-Open No. 06-178244 describes a technique for performing correction for each display panel (or apparatus) by using a correction circuit provided for each display panel. Japanese Patent Laid-Open No. 2002-072359 describes a technique in which a single correction circuit is used to perform correction for multiple display panels.
However, the technique described in Japanese Patent Laid-Open No. 06-178244 requires a correction circuit to be provided for each display panel, making it difficult to reduce the circuit scale.
In the technique described in Japanese Patent Laid-Open No. 2002-072359, before correction is performed for display panels, a whole image is stored in a buffer and then the image is divided into as many partial images as the number of display panels. Correction is then performed for each divided image by using the single correction circuit. Therefore, the method in Japanese Patent Laid-Open No. 2002-072359 incurs high latency between the reception of an image by the HMD and actual display of the image. Especially in MR technology, the latency required for displaying an image on an HMD must be minimized. In this respect, the method described is disadvantageous.
As described above, it is difficult to minimize the circuit scale with the technique described in Japanese Patent Laid-Open No. 06-178244 in which a correction circuit is provided for each display panel. It is difficult to reduce the latency required for displaying an image with the technique described in Japanese Patent Laid-Open No. 2002-072359 because a whole image is temporarily stored in a buffer before the image is divided.